An evanescent-wave optical biosensor includes a hollow optical waveguide, preferably in the form of a light-conductive capillary, surrounding a central waveguide preferably in the form of an optical fiber to create a sealed cavity. A source of optical energy as from a laser is directed into one or both of the light-input ends of the capillary and fiber, such that an evanescent field extends into the cavity from one or both of the inner surface of the capillary and the outer surface of the fiber. A first biomolecular constituent is attached to one or both of the inner wall of the hollow optical waveguide and the outer surface of the second optical waveguide, such that the first biomolecular binding partner is substantially within the evanescent field if present. A first optoelectric detector is supported to receive light from the light-output end of the capillary and convert the light received into a first electrical signal, and a second optoelectric detector is supported to receive light from the light-output end of the fiber and convert the light received into a second electrical signal. A fluid within the cavity which may contain a second biomolecular constituent having a binding affinity to the first biomolecular constituent, such that if binding occurs between the biomolecular constituents, a representative change occurs in the light emerging from one or both of the output ends of the hollow and second optical waveguides and the electrical signals from the optoelectric detectors.